Bugs and Berries: Deciphering the Winter Diet of the Hermit Thrush

January 1, 2011 by Tina Gheen

The availability of food can directly affect the physical condition of birds in winter and can carry over into the breeding season. Discovering how limited food resources affect the condition and health of migratory birds is important to understanding the overall life cycle of these birds.

To further this goal, scientists from the Smithsonian Migratory Bird Center and the University of Georgia took a closer look at the winter diet of hermit thrushes (Catharus gattatus) in South Carolina during the winters of 2005 and 2007.

The team wanted to learn more about how the thrush's diet changed over the course of the winter months. They studied how the amount of arthropods and fruit in the thrush's diet varied throughout the winter and measure how the birds' diet changed because of the availability of food.

The hermit thrush spends its winters in the southeastern U.S., Mexico, and Guatemala and depends on a diet of both arthropods and fruit to sustain itself during the non-breeding, winter months. During the summer, hermit thrushes primarily forage for arthropods, but in colder months, the arthropod population fluctuates depending on temperatures and may disappear altogether.

When this happens, the birds begin to supplement their diets with fruit from plants such as waxmyrtle (Myrica cerifera), inkberry (Ilex glabra), farkleberry (Vaccinium arboreum), yaupon holly (Ilex vomitoria), and beautyberry (Callicarpa americana).

Since it is difficult to measure the actual amount of food birds eat in the wild, the team used stable carbon and nitrogen isotopes to help identify changes in the birds' diet. Stable carbon isotope levels indicated an unchanging diet. Lower levels of carbon isotopes in blood samples indicated the birds were eating more fruit, and higher levels indicated they were eating less fruit.

Scientists took blood samples for the tests from captured birds in December, January, February, and March. But the team did not rely on these tests alone. As further verification of the shift from a diet of arthropods to one primarily of fruit, the team studied fecal samples of the captured thrushes for further clues as to what the birds were eating. Using these tests in combination with a survey of the vegetation and arthropod populations in the study area during early, mid-, and late winter, the team was able to uncover several interesting patterns.

The team found the carbon and nitrogen isotope signatures in the birds' blood samples were consistent with the availability of fruit and arthropods measured in both winter seasons. More enriched carbon isotope signatures, such as those found in late winter, usually indicated lower fruit consumption and higher arthropod consumption.

The birds ate more fruit in the early part of winter (lower carbon isotope signatures) but shifted to a diet heavy in arthropods later in winter and early spring when fruit availability decreased and arthropod availability increased again (higher carbon isotope signatures).

However, the study site in South Carolina had an unusually cold winter in 2005. Consequently, the wildlife in the area consumed the available fruit much faster in 2005 than in 2007. The availability of fruit declined by 98 percent from November to January in 2005 compared to 73 percent in 2007.

The scientists found the carbon isotope levels were reduced in midwinter of 2005, which suggested the birds ate more fruit as arthropod populations declined and the birds needed more energy to keep warm. The team also found the majority of the January 2005 fecal samples contained fruit compared to only 33 percent of samples in January 2007 when temperatures were warmer.

The scientists also found that larger male thrushes typically occupied territories with the highest concentrations of arthropods. Since the diet of these larger birds contained more arthropods, their carbon isotope levels and fat loads were more stable over the course of winter. The smaller predominantly female thrushes in the lower quality habitats spent more time foraging to increase their fat reserves midwinter, which exposed them to additional threats from predators and possible starvation.

Although arthropod availability fluctuated with temperatures throughout the winter, the team found they were an important part of the hermit thrush diet. Arthropods were preferred over fruit by the thrushes, and larger birds consistently controlled the territories with the most arthropods.

However when arthropods were not readily available, usually in the coldest part of winter, the birds used fruit as a crucial supplement to increase fat stores and maintain energy levels. Unfortunately, fruit does not provide the same nutrients as arthropods, and the amount of fruit available to birds consistently decreases as the season progresses, but it certainly helps them get through a hard winter.

This article summarizes the information in this scientific paper:

Diggs, Nora E., Marra, Peter P., and Robert J. Cooper. 2011. Resource Limitation Drives Patterns of Habitat Occupancy during the Nonbreeding Season for an Omnivorous Songbird. The Condor 113(3):646-654.